The present invention relates in general to the printing industry and more particularly to a method of making overlay masks for photoengraving printing plates.
Present day printing processes which produce high quality pictorial or other printed matter on fine paper, such as found in magazines, catalogues and other publications prepared in large numbers, may require as many as eight or more overlay masks to produce a single printing plate. The masks separate printed matter on various portions of a page and are also used for a number of other purposes.
For example, if a grey-tone picture is to appear on a portion of a page together with other word matter, it may be necessary to expose the printing plate at least twice in the photoengraving process, once to generate the grey-tone images from a halftone transparency, and again to expose the word matter. During each exposure, those portions of the page which are reserved for other material may not be exposed and hence are covered by means of an overlay mask having opague portions in a pattern corresponding to the reserved areas. Additionally, halftone plates for both the grey and colored pictures must be cropped in order to define a sharp edge pattern. Masking is particularly important in the case of color separation halftone plates since the masking insures that each color halftone has identical borders and the borders all fall into registration when printed.
Overlay masks also are utilized to produce framing and other line work which divide various areas of a page into sections, or frame pictures and word matter. For example, a vertical bar dividing a page into a right-hand and a left-hand portion may be photographically etched in a printing plate by means of a mask that is opague except for the bar itself. Such a mask may be trimmed and pasted together with other masks defining the word matter, and a single exposure of the printing plate with the pasted masks is then possible.
A frame such as a rectangular frame surrounding a picture or other printed matter is sometimes produced by means of two masks, one having an opague rectangular area and transparent surroundings, and the other having a transparent rectangular area of slightly larger size surrounded by an opague area. When two such masks are positioned together in proper registration over a photosensitive surface of a film or plate, only the transparent area between the two rectangles of different size is exposed on the sensitive surface to form a frame.
Making overlay masks for printing plates in the past has largely been a manual process. A rubylith sheet material consisting of a transparent backing coated with a strippable layer of opague material is stripped in accordance with the desired mask pattern so that the opague material remains in those areas where no exposure of the photographic plate or film is desired. The stripping of the opague material from the transparent backing is done by cutting the outline of a line, frame or box with a sharp blade which only penetrates the opague material and leaves the transparent backing in tact. The cutting of a thin bar or frame requires substantial effort since not only the dimensions of the outline must be accurately controlled, but also the location of the outline may have to be coordinated with other masks. For example, if a frame is desired about a halftone picture with no spacing between the edge of the halftone and the frame, the inner edge of the frame must correspond precisely with the "knock out" box or window prepared in another mask for the halftone picture. Also, if two halftone pictures must abut one another with no space between, the "knock out" windows for each picture are produced in two separate overlay masks, and precise correlation of the windows in each mask is needed at the common edge of the pictures.
The above-described examples are merely a few of many masking techniques which require precise, accurately prepared overlay masks for producing printing plates.
It is an object of the present invention to provide a new process for making overlay masks by means of a precisely controlled photoplotter.